The present invention relates to a total control system of power trains in which an engine having a plurality of output characteristics with respect to a throttle opening is combined in tandem with an automatic transmission having a plurality of transmission gear ratios.
The aforementioned power train is applied, for example, to a power unit of a vehicle. As an engine which can be applied to the power unit, Japanese Patent Application Laid Open No. 3-111610 discloses a so-called variable valve type engine which includes a plurality of cams for operating suction and exhaust valves so that the output characteristics can be changed over with respect to the throttle opening.
This engine is provided with 3 kinds of cams for suction and exhaust valves. As shown in the valve lift characteristic diagram of FIG. 9, these cams include a fuel consumption cam (referred to as "P cam", hereinafter) in which importance is attached to fuel consumption in the case of a partial-load operation, a low speed cam (referred to as "L cam", hereinafter) that is useful in the case of low speed operation, and a high speed cam (referred to as "H cam", hereinafter) that useful in the case of high speed operation. These cams are selectively used in accordance with the present operating condition of load and speed of the engine. As illustrated in FIG. 10 in which output torque characteristics in the case of full throttle are shown, the output characteristics of the engine can be changed over according to the cam to be used, which is selected from P, L and H cams.
On the other hand, the automatic transmission is usually structured as in RE4F02A type automatic transaxle developed and utilized by the assignee of this application. For example, as shown in FIG. 11, according to the speed change pattern that has been previously determined in the form of a two dimensional map of vehicle speed and throttle opening, an appropriate speed change ratio (the first to fourth speed change ratio) is determined from the vehicle speed and throttle opening, when the determined speed change ratio is different from the present one, the transmission gear is changed.
However, when the engine output characteristics changeover control and the speed change gear changeover control are individually conducted in the power train in which the engine and automatic transmission are connected in tandem in the manner of the aforementioned conventional example, it is the same as a case in which 3 kinds of engines having-different output characteristics are combined with the same automatic transmission. Therefore, a predetermined performance can be obtained only at a specific output characteristics, and further the speed is changed when the output characteristic is changed over. Due to the aforementioned speed change, the output characteristics is changed over, so that both control operations affect each other, and a stable operating condition cannot be provided. Therefore, what is called "cam hunting" and "shift hunting" are caused. Accordingly, the driving property deteriorates, and further it is disadvantageous from the viewpoint of improving fuel consumption and exhaust gas emission characteristics.